1. Field of the Invention
The present invention relates generally to a narrowband rejection filter including a waveguide having an azimuthally asymmetric grating. More specifically, the present invention is directed to a single-mode depressed inner clad, photosensitive matched inner clad or photosensitive depressed inner clad optical fiber design having an azimuthally asymmetric photoinduced Bragg grating that couples out of a forward propagating LP.sub.01 core mode over a narrow wavelength band while practically suppressing backreflection. The present invention further relates to system applications of the novel filter design.
2. Description of Related Art
An optical fiber typically includes a core region of refractive index n.sub.co or n.sub.1. In double clad fiber designs, the core region is surrounded with an inner cladding having a refractive index n.sub.ic or n.sub.1a, which is in turn surrounded by an outer cladding region having a refractive index n.sub.oc or n.sub.2. The outer cladding is surrounded by an external medium having a refractive index n.sub.ext. A region of the fiber may be photosensitive.
The free parameters in double cladding waveguide designs are defined as follows:
MFD operational mode field diameter PA1 .lambda..sub.o operational wavelength PA1 .lambda..sub.c second mode cutoff wavelength PA1 A, r.sub.co core radius generated from a single effective step approximation of the core region PA1 W inner cladding width generated from a single effective step approximation of the inner cladding region PA1 B, r.sub.oc outer cladding radius PA1 n.sub.co, n.sub.1 core refractive index generated from a single effective step approximation of the core region PA1 n.sub.ic, n.sub.1a inner cladding refractive index generated from a single step approximation of the inner cladding region PA1 n.sub.oc, n.sub.2 outer cladding refractive index PA1 n.sub.ext external medium refractive index PA1 R.sub.grat maximum radius of photosensitive region of the fiber PA1 .gamma. fractional photosensitivity of photosensitive inner cladding as compared to the core, .gamma.=(grating strength in cladding)/(grating strength in core). (For example, .gamma.=1 means equal photosensitivity in core and inner cladding.)
"Grating tilt angle" is defined as the angle between the grating vector (the direction normal to the planes which define the grating periodicity) and the longitudinal axis of the fiber, where a physical tilt of the grating planes exists in the fiber.
"Effective grating tilt angle" is defined as the value of "grating tilt angle" that is shown experimentally to give cladding-mode loss equivalent to the cladding-mode loss when an azimuthal asymmetry other than a physical tilt of the grating fringes is present in a grating inscribed in the waveguide.
"Fundamental rejection notch" is defined to be the feature or dip in the transmission spectrum of the waveguide grating with the smallest value of transmission. Generally, this corresponds to the wavelength for which a reduction in the transmitted signal is desired.
"Cladding mode losses" refers to losses or dips in the observed transmission spectrum for a waveguide containing a grating that occur due to coupling of the forward propagating modes of the waveguide to counter-propagating bound cladding modes (in the case of n.sub.ext &lt;n.sub.oc), or counter-propagating radiation modes (in the case of n.sub.ext .gtoreq.n.sub.oc). Peak cladding mode loss ("PCML") is the maximum "cladding mode loss" in a given transmission spectrum for a waveguide containing a grating.
A passive component that couples light out of the forward propagating LP.sub.01 core mode of a single-mode telecommunications fiber with relatively negligible backreflection over a narrow wavelength band is a critical filtering element for lightwave systems in which no backreflected signal can be tolerated.
The standard approach to filtering has been to couple the forward propagating LP.sub.01 mode to a counter propagating LP.sub.01 mode. Conventional Bragg gratings redirect the filtered signal back into the fiber. The higher the filtering ability of a traditional Bragg grating, the greater is the back reflection. However, in many systems, such as WDM applications, the back reflection may cause deleterious effects, such as destabilizing the signal emitting laser. Presently, an available alternative is to install isolators, which protect sensitive equipment. However, the current cost of isolators is high.
Work towards obtaining a narrowband rejection filter utilizing a grating in which the loss is not obtained through coupling to the counterpropagating LP.sub.01 core mode can be broken into two categories: coupling to the LP.sub.11 mode in a two mode fiber and coupling to cladding or radiation modes in a single-mode fiber.
Suppression of the forward propagating LP.sub.01 core mode over a narrow wavelength band was shown in a two mode fiber. The backreflected signal at the peak rejection wavelength due to coupling to the counter-propagating LP.sub.11 core mode was measured to be -15 dB, a value smaller than the commonly desired isolation for wavelength division multiplexing applications. In addition, two-mode fiber filters may have higher insertion losses than filters in single-mode fiber due to an imperfect mode-field match. Standard telecommunications fibers usually are single mode fibers.
Attempts towards suppression of the core mode using single mode fibers have discussed how in a strong, tilted grating, significant coupling to the LP.sub.1m modeset could occur. Additionally, outcoupling light from a single-mode fiber utilizing a transversely asymmetrically shaped Bragg grating has been attempted. However, these attempts have failed to couple a significant fraction of power from the fundamental mode.
The need remains for a filter design that allows for coupling out of the LP.sub.01 core mode with near, if not complete, suppression of coupling to the backward propagating fundamental mode.